question

internal or external BMS installed ? ...

Looks like it is a dc/dc Booster like the victron Orion ... Try to messure the Voltage on the Input ... I think that your 12V "Carside" goes down and the booster switch to Safety Mode ... and come back again ... How much Current does your Generator have?

@Newbflat. I have the same issue with my 1250D. Mine is installed in a Landcruiser 200 charging a 100Ah Lithium, and I also have a 170W Solar on the roof.
After discussions with Redarc (who blame the Victron BMV for not been quick enough to accurately read the V & A values - see discussion below) I eventually concluded that it is an issue to do with their so called 'Green Power' capability of the device. Basically the unit will take what ever Solar power is available first, and top this up to the maximum output power using your car alternator.

What I discovered is that on bright sunny days, with the solar panel able to operate with a steady output, the BCDC unit output a stable Voltage and Current close to its rated output (I was getting around 48A). However on cloudy overcast days, I would get the severe saw toothing output that you show in your picture. If I immediately blocked the solar panel from the sun (drive under carport), the output of the unit would instantly stabilise.

So my conclusion is/was, that on very low output currents of the solar panel (ie in very poor sun) the BCDC unit is struggling to get a stable MPPT signal and constantly switches between 2 modes 1) all power from the alternator, and 2) trying to grab some from the solar panel with alternator top up.

My solution is to put an ignition switched relay on the solar input line so that when the car is on, No solar is available. I asked myself what is the point of trying to add a tiny bit of solar power to replace some alternator power - answer none.

Below these comments I have included the Redarc reply to my query.

You should note, they try to tell me the unit is current limiting to protect the battery. Firstly it's a Lithium and its rated to up to 100A charge current. Secondly the unit is only outputting around 36A, so nowhere near its maximum.

I called them on this, and showed that it was related to the solar panel in full/overcast conditions.

They didn't really relent on their belief that it is a BMV monitoring issue, but did offer to change my unit over to another unit with later firmware. (for your information - REDARC gear is not firmware upgradable for end users).

I took them up on the offer, but the replacement unit misbehaved exactly the same, so I now simply block the Solar panel.

This post is not a put down of REDARC BCDC chargers, they are a very compact weather proofed unit (can be installed in the engine bay) come in a range of outputs, and include a very good MPPT controller when operating with Solar only. Its the attempt to go GREEN I think is the issue.

REDARC Reply

Thank you for your enquiry.

What you are seeing is based on the timing of sample points, rather than an RMS current output.

As the BCDC reaches current limit for example it will drop off and then ramp back up again in order to charge the battery without damaging itself or providing too much current, and the reading your meter is showing is a very “slow” look at what is actually occurring.

The RMS current output throughout this process is actually somewhat visible in the graph, but cannot really be viewed accurately without an RMS meter (line drawn on graph is close to accurate). Once this current limiting is no longer required, you see the graph then smooth out.

Now adding the MPPT solar factor into this, and you have quite a complex variation that may look rather erratic based on the sample points the meter is showing, as the MPPT regulator is also searching up and down the solar voltage range for the maximum power point (the BCDC engineer said this is very common viewing on the Victron meter and was not at all surprised or concerned for your charger!).